This invention relates to an optical frequency synthesizer and/or sweeper (referred to as synthesizer/sweeper), which emits coherent light having its frequency, phase, amplitude and polarization all selectively controlled.
2. Description of the prior art.
A conventional laser light source having a wavelength sweeping function may be one of the following types which are depicted in FIGS. 1 and 2.
(A) In one type, the temperature properties of a wavelength of a semiconductor laser are utilized, and the wavelength is swept by varying temperature of a laser diode. FIG. 1 is an explanatory view showing its principle. A constant temperature oven TB is controlled by a temperature control means TC, thereby sweeping an output wavelength of a laser diode LD. In this case, the sweeping width comes to several tens of nanometers.
(B) In another type, a prism is provided within a resonator and rotated and an oscillation wavelength is swept by making the most of a large width of gain of a coloring laser. In FIG. 2, which describes such other type, there is provided a mirror M, a coloring cell CC, a lens LS, a prism P, and a half mirror HM. The sweeping width is approximately 100 nm.
These tunable laser light sources are, however, disadvantageous. For example, the accuracy of wavelength is undesirably at most 1 nm (at 300 GHz). In the fields of prospective coherent light communications and optical photo-applied measurements, frequency measurement with accuracy of less than the level of MHz is required. Hence, the above described laser light sources are not useable as coherent light measuring devices.